WASHINGTON — In a significant setback for President Obama’s signature domestic initiative, the administration on Tuesday abruptly announced a one-year delay, until 2015, in his health care law’s mandate that larger employers provide coverage for their workers or pay penalties. The decision postpones the effective date beyond next year’s midterm elections.

Employer groups welcomed the news of the concession, which followed complaints from businesses and was posted late in the day on the White House and Treasury Web sites while the president was flying home from Africa. Republicans’ gleeful reactions made clear that they would not cease to make repeal of Obamacare a campaign issue for the third straight election cycle.

While the postponement technically does not affect other central provisions of the law — in particular those establishing health-insurance marketplaces in the states, known as exchanges, where uninsured Americans can shop for policies — it throws into disarray the administration’s effort to put those provisions into effect by Jan. 1.

“I am utterly astounded,” said Sara Rosenbaum, a professor of health law and policy at George Washington University and an advocate of the law. “It boggles the mind. This step could significantly reduce the number of uninsured people who will gain coverage in 2014.”